With the development of the information industry, the technology that can transmit various kinds of large data at high speed has been required. In this respect, a DAS that provides a plurality of distributed antennas within an existing cell has been studied to remove a shade zone and extend coverage.
The distributed antenna system (DAS) uses a plurality of distributed antennas connected with a single base station through a cable or a dedicated line, wherein the single base station manages a plurality of antennas located within a cell at predetermined intervals. The DAS provided with a plurality of antennas located within a cell at predetermined intervals is different from a centralized antenna system (CAS) provided with a plurality of antennas centralized at the center of a cell. Generally, the CAS uses various multi-antenna systems such as an open loop-multi input multi output (OL-MIMO) system, a close loop-single user-multi input multi output (CL-SU-MIMO) system, a close loop-multi user-multi input multi output (CL-MU-MIMO) system, and a multi-base station-multi input multi output (multi-BS-MIMO) system in such a manner that multiple antennas are installed in one base station in a cell based structure of a cellular communication system such as a wideband code division multiple access (WCDMA) system, a high speed packet access (HSPA) system, a long term evolution (LTE)/long term evolution-advanced (LTE-A) system, and a 802.16 system.
The DAS is different from a femto cell in that all distributed antennas located within a cell are managed by a base station at the center of the cell not each unit of the distributed antennas. Also, the DAS is different from a multi-hop relay system or ad-hoc network in that distributed antenna units are connected with one another through a cable or a dedicated line, wherein the multi-hop relay system or ad-hoc network includes a base station connected with a relay station (RS) through a wireless mode. Moreover, the DAS is different from a repeater, which simply amplifies and transmits a signal, in that each of distributed antennas can transmit different signals to different user equipments located near the antennas in accordance with a command of a base station.
This DAS may be regarded as a multiple input multiple output (MIMO) system in that distributed antennas can support a single user equipment or multiple user equipments by transmitting and receiving different data streams at the same time. In view of the MIMO system, the DAS is advantageous in that it can reduce a transmission power as a transmission zone is more reduced than that of the CAS due to distributed antennas distributed at various locations within a cell. Also, the DAS can reduce path loss by reducing a transmission distance between the antenna and the user equipment, thereby enabling high rate transmission of data. As a result, the DAS can enhance transmission capacity and power efficiency of the cellular system, and can satisfy communication throughput of quality more relatively uniform than that of the CAS regardless of user location within the cell. Also, since the base station is connected with a plurality of distributed antennas through a cable or a dedicated line in the DAS, signal loss is reduced, and correlation and interference between the antennas are reduced, whereby a high signal to interference to noise ratio (SINR) can be obtained.
As described above, in order to reduce facility costs of the base station and maintenance costs of a backhaul network in a next generation mobile communication system and at the same time extend service coverage and improve channel capacity and SINR, the DAS can be a new basis of cellular communication by being compatible with the existing CAS or replacing with the CAS.